This invention relates to an electron emitter for microwave tubes and more particularly to a micro-patch emitter with an integral control grid positioned on the cathode surface relative to equally spaced pores or openings through which low-work-function material effuses and migrates onto the cathode surface adjacent the grid structure.
Modern microwave tube designs are concentrating on applications involving ever-higher frequency and power requirements. Since the electron emission capabilities of cathode surfaces are limited, and the electron-beam interaction space becomes smaller with increasing frequencies, it is seen that the maximum convergence ratio (cathode area/beam area) which can be used strongly affects the limits of both the power and frequency of these designs. A main factor which restricts the convergence ratio is the beam spreading caused by excessive transverse electron velocities such as caused by grids which are used to control or modulate the electron beam. In order to minimize the effect of transverse velocities, as well as decrease the modulation voltage (which reduces modulator power and size requirements as well as increases fast switching and electrical turn-on performance), recent cathode designs have utilized grids which are in very close proximity, or actually are bonded directly to the cathode. However, in order to maintain good grid cut-off characteristics of the tube (as is required for example, during the time between pulses of "quiet" radars), it is necessary to maintain the non-emitting properties (i.e., high work-function) of the grid. Unfortunately, the close proximity of the grid to the hot cathode makes this task difficult because the grid is heated by the cathode to elevated electron emission temperatures and also because of the evaporation of the low-work-function material onto the grid from the cathode, which acts to lower the work function of the grid.
U.S. Pat. No. 4,096,406 sets forth a thermionic electron source with bonded grid control. This patent uses a barium aluminate porous tungsten cathode from which BaO effuses through pores in the tungsten. The grids are bonded to the cathode structure but during the bonding process the grid covers some of the pores so that some areas between the grids are free of BaO. This limits the operation of the device.